This invention relates generally to pressure regulating valves, and more specifically to pressure proportioning valves that are operable to maintain the outlet pressure a differential below the inlet pressure after a predetermined inlet pressure has been reached or exceeded. Still more specifically, this invention relates to such pressure proportioning valves that are particularly adapted for use in motor vehicle hydraulic brake systems.
In motor vehicle hydraulic brake systems, it is sometimes desirable to provide a lower fluid pressure to the rear wheel brakes than to the front wheel brakes under severe braking conditions. This prevents permature lock-up of the rear wheel brakes, which might otherwise be caused by a weight transfer from the rear wheels to the front wheels upon rapid deceleration. It is also desirable, particularly when the motor vehicle is a two axle truck which has a significantly greater load on the rear wheels when the truck is loaded than when it is empty, to provide a different relationship between front and rear wheel brake pressures when the vehicle is fully loaded than when the vehicle is empty.
Two prior art proportioning valves which maintain the rear brake pressure below the front brake pressure and which provide different pressure reductions for a fully loaded vehicle than for an empty vehicle are disclosed in U.S. Pat. No. 3,773,362. The two valves disclosed in this patent provide a rear brake pressure which is proportional to but less than the front brake pressure over one range of operation and then cut off all further increases in rear brake pressure.
Another type of pressure proportioning valve is disclosed in U.S. Pat. No. 3,467,440, which discloses a valve which follows one pressure reduction curve under normal operations and which follows a different pressure reduction curve when there is a pressure failure in the front wheel brakes. Additionally, pressure limiting devices which terminate further increases in braking pressure when a predetermined deceleration rate is reached are known such as disclosed in U.S. Pat. No. 3,403,946.